German Offenlegungsschrift 4,031,848 discloses a process for the preparation of crystalline sodium silicates having a laminar structure, a molar ratio of SiO.sub.2 to Na.sub.2 O of (1.9 to 2.1) : 1 and a water content of less than 0.3% by weight from a water glass solution having at least 20% by weight of solids. In this case the water glass solution is obtained by reaction of quartz sand with sodium hydroxide solution in the molar ratio of SiO.sub.2 to Na.sub.2 O of (2.0 to 2.3) : 1 at temperatures of 180.degree. to 240.degree. C. and pressures of 10 to 30 bar. This water glass solution is treated in a spray drying zone with hot air at 200.degree. to 300.degree. C. with formation of a pulverulent amorphous sodium silicate having a water content (determined as loss on heating at 700.degree. C.) of 15 to 23% by weight and a bulk density of more than 300 g/l. The pulverulent, amorphous, water-containing sodium silicate is introduced into an inclined rotary kiln equipped with devices for moving solids and is treated therein with flue gas at temperatures of more than 500.degree. to 850.degree. C. for 1 to 60 minutes in counter-current flow with formation of crystalline sodium silicate. Finally, the crystalline sodium silicate leaving the directly heated rotary kiln is comminuted with the aid of a mechanical crusher to particle sizes of 0.1 to 12 mm.
A disadvantage of the known process is the difficult establishment of a reproducible temperature profile in the rotary kiln because of the variations of the flue gas with respect to its quantity per unit time and its temperature, by which means crystalline sodium disilicate of varying quality is produced.